There are known inorganic materials (oxide ion conductors) having the property of selectively transmitting oxide ions at high temperature (such as 500° C. or higher). Sinters of multi-component metal oxides including lanthanum or lanthanide (hereinafter also referred to as “lanthanum-based oxide ion conductors”), such as an LaSrCoO3-based composite oxide or an LaGaO3-based composite oxide, are known as oxide ion conductors with particularly high oxygen transmission performance. These lanthanum-based oxide ion conductors can be utilized in applications such as separating oxygen from a mixed gas containing oxygen. Some oxide ion conductors are both oxide ion conductive and electron conductive (the meaning of which includes hole conductivity). Such oxide ion conductors are also called electron-oxide ion mixed conductors (hereinafter also referred to simply as “mixed conductors”).
Types of lanthanum-based oxide ion conductors, and how they are used, are discussed, for example, in the specifications of Japanese Laid-Open Patent Applications 2001-106532, 2001-93325, 2000-154060, H11-335164, H11-335165, H10-114520, H9-299749, and S 92103, Japanese Patents 2,993,639 (Japanese Laid-Open Patent Application H11-253769), 2,966,341 (Japanese Laid-Open Patent Application H9-235121), 2,966,340 (Japanese Laid-Open Patent Application H8-276112), 2,813,596 (Japanese Laid-Open Patent Application H6-219861), and 2,533,832 (Japanese Laid-Open Patent Application H6-198149), and U.S. Pat. Nos. 5,306,411 and 5,356,728.
Lanthanum-based oxide ion conductors are generally manufactured by a solid phase reaction method (see the examples (manufacturing examples) in the various publications listed above). This is because the cost is higher and fewer types of starting raw material can be used with a liquid phase reaction method.
With a solid phase reaction, a mixed powder is prepared by mixing a number of types of metal compound (oxides or various salts) so that all of the elements constituting the oxide ion conductor will be included. The mixed powder is then prefired within a predetermined temperature range. Molding the prefired powder thus obtained into a predetermined shape and firing it (hereinafter referred to as “main firing”) yields an oxide ion conductor (sinter) of the desired shape.
In order to ultimately obtain a dense sinter (that is, a sinter with a structure dense enough to ensure gas impermeability) in the manufacture of an oxide ion conductor (sinter) by means of a solid phase reaction, it is preferable for the raw material powder that makes up the molded article subjected to the main firing to have higher activity (sintering reactivity). Accordingly, the temperature at which the mixed powder is prefired is set relatively low (800 to 1000° C., for instance).
However, while the activity (sintering reactivity) of the raw material powder will be high if the mixed powder prefiring temperature is low, unreacted particles that have not undergone prefiring (hereinafter referred to as “impurities”) tend to remain behind in the powder. These impurities can bring about a chemical reaction that leads to a significant change in volume during the main firing of the molded article. For example, they can bring about a volumetric change in which first swells and then contracts under predetermined conditions.
If such impurities are present in a large quantity in the prefired powder, cracks tend to develop in the oxide ion conductor (sinter). Cracking is undesirable in an oxide ion conductor, because gas can pass non-selectively through these cracks and thereby through the oxide ion conductor, the result of which is a decrease in the selective oxide ion permeation (separation) performance of the oxide ion conductor.
The present invention was conceived in an effort to solve the above problems encountered in the past in relation to a raw material powder (prefired powder) manufactured by solid phase reaction method. It is an object thereof to provide a raw material powder for stably obtaining a dense sinter (oxide ion conductor) that is prevented from cracking, and a method for manufacturing this powder. It is a further object of the present invention to use this raw material powder to manufacture lanthanum-based oxide ion conductors of various shapes.